In the southeast corner of the London Borough of Waltham Forest, a little project is nearing completion which is designed to alleviate a local flooding problem.
Esther Road is an unremarkable street in the north of Leytonstone, a community which was severed by the A12 in the 1990s as part of the M11 Link Road project, one of the last large road schemes in London of that era and which was bitterly protested against.
What makes the street more interesting than the others in the area is some sections of the car parking on both sides have been repurposed for surface water management using a Sustainable Drainage Systems (SuDS) feature commonly known as "rain gardens" (above).
The project covers three streets in the area which were hit by floods in 2021 and which are in a bit of a valley circled in red above - taken from the London Topographic Map. It's a problem which is only going to get worse with climate change and the project demonstrates that we're going to need lots of these little localised interventions in order to adapt to the warmer wetter conditions the UK faces.
In terms of design, the layout is pretty common whereby the rain garden areas are bounded by kerbs to retain the adjacent carriageway and there are regular slots to take the flows from it. In theory, the kerb could be flush with the road surface, but usual practice is to keep an upstand to dissuade drivers and to provide a detectable edge should someone walking require it (above).
A closer look along the rain garden shows the existing kerbline (left) and new kerbline (right) supported with concrete backing, and what is nice here is the edge of this concrete has been neatly formed and faced with stones set into it. These stones not only look nicer than formed concrete, they'll also do a little bit to dissipate the energy of water entering the rain garden.
Elsewhere, we can see chamber covers set into the rain gardens and curious red domes (above). I'm not entirely sure how this all fits together for this project, but the covers will provide maintenance access to chambers which provide the ultimate connection to the sewer network.
The red domes are overflows (above) with a pipe extending downwards. Designs vary, but the general arrangement of a rain garden like this will be that water enters at the surface and soaks into the soil and then down into a stone layer and the ground itself.
The stone layer is specially graded to provide 30% voids and so provides storage for flood water. If the inundation exceeds the ability of the surrounding soils to absorb, then the stone layer fills with water, then the soil layer fills and then it finally overflows to the sewer. The soil layer is also specially selected for the job to be free-draining, and in fact, rain gardens need plants which can cope with long dry spells!
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| © City Infinity |
In ground where water infiltration is likely to be very slow, the bottom of the stone layer will have a pipe (called an underdrain) with lots of little holes to allow water to percolate into and drain away after the storm event, otherwise the stone layer will stay saturated and so be useless if another storm event occurs in a short time. Above is a typical detail of how that would look in long section - there will also be a layer of geotextile around the stone layer (a cloth-like material) which stops fine particles getting in and clogging the voids needed for water storage.
The project isn't just about the rain gardens, it includes a trial of rainwater planters for households as a way of keeping even more water away from the sewers, but there is also something in plain sight that unless you knew about it, you might miss. Have a look at this little video.
Yes, the water is soaking into the asphalt surface! The product is "SuperDrain" and is designed to allow water to drain through; and like the rain gardens, the substructure of the road has been rebuilt with stone layers designed to retain flood water with outfall structures designed to slowly release the water into the sewer system - I don't know the exact details, but I assume there is an underdrain of some sort.
The porous carriageway is an expensive thing to build, but it could be used for new-build developments. For Esther Road, it has been used to deal with an existing problem and in an urban area, we don't have the space to build ponds and other land-hungry solutions so we need to use existing streets. That to one side, rain gardens are a cost-effective measure which can be built all over and it is this kind of diffuse solution that can provide resilience if deployed routinely.
For more technical information on rain garden design, it is well worth looking at Designing Rain Gardens: A Practical Guide from Urban Design London, although if used outside of London, you need to use appropriate local storm intensity information - one for designers!